This invention relates to nuclear medicine, and more particularly relates to SPECT studies which are carried out to form tomographic images of the uptake of radioisotopes within body organs. In its most immediate sense, the invention relates to attenuation correction of data used in SPECT studies of, e.g., the heart.
In a conventional SPECT (Single Photon Emission Computed Tomography) study of an organ such as the heart, a radioisotope (Tc-99m, T1-201, for example) is administered to the patient and the radioisotope is taken up by the heart muscles. Then, the patient is placed in a scintillation camera system and one or more scintillation camera detectors are rotated about the long axis of the patient. These detectors pick up gamma radiation which leaves the patient, and the resulting data is used to form three-dimensional images ("SPECT images" or "tomographic images") of the distribution of the radioisotope within the patient.
Such three dimensional SPECT images can be calculated based on a set of two-dimensional images ("projections" or "projection images") acquired by the scintillation camera system; this calculation process is known as image reconstruction. The most commonly employed method of image reconstruction is known as "filtered backprojection". When filtered backprojection reconstruction is used to reconstruct SPECT images from scintigraphic projection images obtained from a scintillation camera, some well-known distortions introduce errors ("artifacts") in the result. One of the most important distortions is caused by attenuation of gamma radiation in tissue.
As a consequence of attenuation, image values in the various projections do not represent line integrals of the radioisotope distribution within the body. It is therefore necessary to correct for this, and the process for doing so in SPECT is known as attenuation correction.
Many techniques for attenuation correction in SPECT assume that the linear attenuation coefficient of the body is uniform and impose such uniformity as a mathematical constraint in the image reconstruction process. However, for a very important class of studies, namely cardiac SPECT studies, the linear attenuation coefficient of the body is in fact highly nonuniform. This is because lung tissue has a lower attenuation than do, e.g., the blood and other non-lung tissue.
Thus, in SPECT studies of, e.g., the heart, a SPECT reconstruction of the image of radioactivity within the heart will necessarily contain artifacts caused by the unequal attenuation coefficients of, e.g., the lungs and the body (and, in the case of some female patients, large breast size.)
It would therefore be advantageous to provide a method for mapping a slice of a patient's body into regions of different attenuation coefficients, thereby permitting the reconstruction process to be carried out on the basis of the patient's actual body structure and without the need to use an overly simplistic modelling assumption, namely, that the entire body has a single attenuation coefficient.
It would further be advantageous to provide a such a method which would actually permit such different attenuation coefficients to be computed, thereby permitting the backprojection process to be carried out on the basis of the actual attenuation coefficients in the patient under study.
It would additionally be advantageous to provide such a method which would add little if any time to the duration of a conventional SPECT study, thereby not diminishing patient throughput through the camera.
In accordance with the invention, the scintillation camera system is set so that Compton-scattered events fall within an energy window, and a scatter image is formed from Compton-scattered events which are acquired during a SPECT study. Because Compton-scattering of gamma rays is the primary attenuation process in body tissue at energies of interest in nuclear medicine, the scatter image will reflect, in a general way, regions of differing attenuation coefficients. In further accordance with the invention, this scatter image is processed so as to define within it bounded regions within which the attenuation coefficient may accurately be treated as constant.
Therefore, in accordance with the invention, a slice of the patient's body is mapped into bounded regions, each region having a single attenuation coefficient. As a result, the backprojection algorithms used in the reconstruction process can take account of the spatial variation in the attenuation coefficient within the patient's body, and need not make the inaccurate assumption that the attenuation coefficient is constant within the body.
In the preferred embodiment, the SPECT event acquisition process takes place with two energy windows simultaneously, one window encompassing only Compton-scattered events and the other window encompassing photopeak events from the radioisotope used in the study. This has the consequence that the study is not prolonged since the scatter image and the conventional nuclear medicine image can be acquired and reconstructed simultaneously, just as can be a conventional dual-isotope study.
Once the scatter image has been formed, it may advantageously be processed in order to more accurately define the boundaries of the regions therewithin. After such processing, the attenuation coefficients of the regions may simply be supplied on the basis of known data, e.g. predetermined lookup tables. Alternatively, the actual attenuation coefficients may be calculated by directing gamma rays through the body and solving a system of equations. This latter procedure might be desirable where predefined tables could be seriously wrong, as in instances of pulmonary congestion.